This invention relates to a photovoltaic conversion element (device) and a dye-sensitizing type photovoltaic cell.
The electric power for actuating a display element (device) which is employed extensively for various end-use now is currently supplied by a battery or cell disposed outside the display element. For example, in the case of a liquid crystal display element, the power is supplied by a cell disposed outside the liquid crystal display element. Therefore, the operating time of such a liquid crystal display is restricted by the life of cell.
If it is desired to extend the operating time of a display element, the employment of solar cell as a power source is effective. However, when the solar cell is to be employed, a light-receiving portion of the solar cell is required to be disposed outside the display element, thereby enlarging the size of the display element.
In view of preventing the display element from being enlarged, there has been proposed to form a solar cell inside the display element. For example, Japanese Patent No. 2728041 describes a method wherein an opaque solar cell such as one comprising Si is disposed at a light-shielding portion of a liquid crystal display element. According to this method however, since it is impossible to secure a wide light-receiving area at the light-shielding portion, it is difficult to supply a sufficient energy to the display element. There is also proposed a method wherein an opaque solar cell is employed as a light absorption layer of liquid crystal display element so as to employ the solar cell as an energy source for actuating the display element (Japanese Patent Unexamined Publication H8-152620). In this case however, since the light absorption layer is placed underneath the liquid crystal layer, the light is shielded by the liquid crystal layer, so that it is also difficult as in the case of the aforementioned method to supply a sufficient energy to the display element.
Further, there is also proposed another method for forming a solar cell inside the display element, wherein a light-transmitting solar cell is formed on the surface of display element. As for a solar cell which is capable of transmitting light, there are known a back surface transparent electrode type solar cell wherein an amorphous silicon solar cell is disposed on a glass substrate, a see-through solar cell wherein fine holes are formed in a silicon substrate thereby allowing light to pass therethrough, or a dye-sensitizing type solar cell. However, since the aforementioned back surface transparent electrode type solar cell is accompanied with the problem that the colors are restricted by the bandgap of silicon, a display element provided with the solar cell is incapable of displaying colors other than red, e.g. blue and green. On the other hand, since the light transmittance is secured by the fine holes in the case of the see-through solar cell, it is impossible to enhance the light transmittance in simultaneous with the enhancement of the supply of energy.
The dye-sensitizing type solar cell is, as described in Japanese Patent No. 2664194 for instance, composed of a first transparent electrode, a transparent semiconductor disposed on the first transparent electrode, a sensitizing dye which is adsorbed on the surface of the transparent semiconductor, a carrier layer formed on the sensitizing dye, and a second transparent electrode disposed over the carrier layer. This dye-sensitizing type solar cell can be operated through the following process.
The light entering the solar cell is allowed to pass through the first transparent electrode and the transparent semiconductor for instance, thus reaching the sensitizing dye. Alternatively, the light entering the solar cell is allowed to pass through the second transparent electrode and the carrier layer, thus reaching the sensitizing dye. Subsequently, this sensitizing dye is excited by the light, thereby generating electrons at the LUMO level and holes at the HOMO level. The electrons of LUMO level of the sensitizing dye that have been generated by the excitation are immediately transferred to the conduction band of the transparent semiconductor, subsequently reaching the first transparent electrode. On the other hand, the residual holes of HOMO level of the sensitizing dye receive electrons from the carrier transport layer, thereby neutralizing the sensitizing dye.
Since the electrons have been delivered, ions or holes are caused to generate in the carrier transport layer. The ions or holes thus generated are allowed to diffuse into the carrier transport layer, thereby reaching the second transparent electrode, thus receiving electrons from the second transparent electrode. The first transparent electrode that has received the electrons functions as a negative electrode, and the second transparent electrode that has handed over the electrons functions as a positive electrode, thereby enabling them to function as a dye-sensitizing type solar cell.
However, according to the conventional dye-sensitizing type solar cell, since only one kind of dye is employed for each of the transparent semiconductor, the external appearance is formed of a single color glass as in the case of the back surface transparent electrode type solar cell. Therefore, it is impossible, in this dye-sensitizing type solar cell, to display a plural kinds of color, so that it is impossible to dispose the dye-sensitizing type solar cell at the interior of display element.
By the way, in the case of a photovoltaic cell using an electrode carrying a dye on the surface of transparent semiconductor layer, a semiconductor electrode having a fine structure which can be obtained by sintering metal oxide fine particles is employed as set forth in Japanese Patent Unexamined Publication H1-220380 or International Patent Publication H5-504023.
In the manufacture of a semiconductor electrode that has been employed for a photovoltaic cell wherein a titanium oxide film is to be employed for instance, first of all, an electrode is dipped into a solvent containing an organic titanium compound such as titanium isopropoxide. Then, after being taken out of the solvent, the electrode is sintered to obtain a semiconductor film. The semiconductor film thus obtained is treated so as to allow a dye to be adsorbed on the surface thereof, and then, interposed between counter electrodes via a liquid carrier transport layer, thereby obtaining a wet type photovoltaic cell.
This photovoltaic cell obtained in this manner can be operated through the aforementioned transfer of electrons and holes. The electromotive force between these electrodes is generated in a magnitude which is reduced by a loss to be brought about as carriers is migrated from the HOMO-LUMO gap of the sensitizing dye to the electrode via the semiconductor layer and the carrier transport layer.
In the case of the conventional photoelectric cell, a semiconductor electrode formed using an n-type semiconductor having a sensitizing dye adsorbed thereon has been employed as a negative electrode, while a metal electrode has been predominantly employed as a positive electrode. Therefore, the electromotive force to be generated within the photovoltaic cell would not exceed over the HOMO-LUMO level of the sensitizing dye of one kind. Thus, the photovoltaic conversion element using only a single band gap or HOMO-LUMO level is limited in photovoltaic conversion efficiency, making it impossible to further improve the photovoltaic conversion efficiency.
On the other hand, the photovoltaic cell comprising a liquid electrolyte as a carrier transport layer is accompanied with a problem of the leakage of electrolyte. Although it is possible to prevent the leakage by using a gel or a solid polymer for the carrier transport layer, since such a gel or a solid polymer is poor in ion conductivity as compared with an electrolyte, the photovoltaic conversion efficiency of the cell would be decreased as compared with that where an electrolyte is employed.
Under the circumstances, it is now desired to develop a cell which is capable of effectively photovoltaically converting a wide range in wavelength of light, thereby making it possible to achieve a higher photovoltaic conversion efficiency.